Over the past few years, the global positioning system (GPS) has enjoyed increased attention being utilized in a variety of different applications requiring precise measurement of location on the surface of the earth. Some of the different applications which make use of the GPS data include: location measuring applications, navigation applications, tracking applications, mapping applications, and timing applications.
Signals sent from GPS satellites may be used by location measuring applications to determine latitude and longitude of a receiving device. GPS receivers may be useful for personal recreation such as hiking, kyacking, skiing, or other activities that may require one to venture to remote locations. Location measuring applications may also be used in moving vehicles such as automobiles or airplanes to determine the present location of any such vehicles, thereby preventing the vehicle's operator from becoming lost. Location measuring applications also find a use in the military, where precision location information is important for targeting, location of enemy forces, or personnel location.
Navigation applications making use of GPS data are useful in moving vehicles and for determining the best path to be taken to reach a desired destination. For example, automobiles may incorporate GPS receivers to determine present location, and use this information in connection with known street layout information to determine the fastest, or most fuel efficient path to a desired destination. Similarly, airplanes may use GPS information in navigating to determine travel information or landing and takeoff information. In the military, GPS navigation applications may be useful in maneuvering blindly, such as at night, or without the aid of lights or other instruments.
Tracking applications using GPS information are useful for monitoring the movement of people and things. For example, the military may use GPS tracking applications to monitor the movement of troops and equipment of enemy forces. Emergency response systems may use tracking applications to determine the present location of emergency medical response teams in an effort to minimize the response time of a team reaching a victim at a desired location.
Mapping applications which utilize GPS signals may be used in cartography for creating more accurate maps. Land surveying and marine surveying may also be enhanced by mapping applications which utilize GPS information. In addition, construction and agriculture may both be improved by mapping applications which utilize precision GPS data for aligning buildings or crops more precisely.
Timing applications which utilize signals from GPS satellites may be used to determine precise timing, and to coordinate time with GPS time and/or universal time (UTC) standards. By so doing, applications such as mobile communications may achieve high levels of timing precision by taking advantage of the precise atomic clocks on board the GPS satellites without incurring the high costs of incorporating atomic clocks.
As can be seen from the above, many advantages are enjoyed by applications using GPS signals and data. This data can be obtained from any of the twenty four satellites currently in the GPS constellation. These satellites are placed in orbit such that a minimum of five are in view from every point on the globe at any given time. Many GPS receivers are configured with an almanac, which allows the receiver to determine the present, expected location of each of the GPS satellites.
GPS is, however, a fairly fragile system having a received power at the GPS receiver location of −160 dB or less. Because of the weakness of the signal, signal forces that are much closer to the receiver's location may cause much unwanted interference. For example, a 5W transmitter can cause potential interference signals that stretch for hundreds of miles. There have been documented cases where people have interfered with the signals received by the airlines with a small transmitter, and where television stations have developed intermodulation frequencies and energies in the process of transmitting their signals that have caused interference for aircrafts. As GPS increases in popularity and availability, the possibility of interference from low-powered transmitter devices becomes a greater concern, and affects more people. Also, as more sensitive systems, such as emergency systems and military applications use GPS data, the prospect of interference from low-powered transmitter devices becomes an area of increasing concern.
Systems which have been developed to date generally use a single, nearly uniform-gain antenna with a hemispherical reception pattern for receiving all signals more than a few degrees above the horizon in all directions. Using a single, nearly uniform-gain antenna allows signals from satellites just above the horizon and nearly overhead to be received with nearly equal power. Many of the improvements to GPS receiving systems have focused on the receiver. Efforts have been made to increase receiver sensitivity so that the relatively weak GPS signals may be received clearly from all directions.
Various other systems have employed multiple antenna elements to utilize a technique known as null steering. Null steering is a method whereby the multiple antenna elements are used to provide cancellation and otherwise reject the interference signals. However, even in rejecting interference signals, the signal strength provided by GPS satellites is weak, and consequently may be easily lost by a GPS receiver. This a problem especially in sensitive applications such as emergency situations, military applications or other critical systems like those employed by the airlines.
Therefore, it is desirable to create a system and method for selectively receiving RF signals such as those from GPS satellites by an array of antenna elements in a manner such that the desired signals are additively combined thereby effectively amplified, while interference signals experience no such amplification.